Drag space option is supposed to add some space to drag window on top of tabs. Currently on Linux it does the following: - If density is set to compact, it makes the blue line of the active tab just a little bit thicker, probably one pixel thicker - If density is set to normal or touch, instead of having that line thicker, it moves it one pixel lower and therefore create a one pixel thick line between tab border and blue line. In every case, no drag space appear.

Tested on Archlinux running Gnome 3.28.

With Arch Linux and KDE Plasma, the behaviour is again somewhat different, - With density set to compact, the drag space is never gone, and enabling "drag space" makes the "active tab indicator line" thicker (same as reporter's) - With density set to normal, if "drag space" is disabled, there's actually always a drag space above tabs; if "drag space" is enabled, after un-maximising and maximising the window, the drag space is gone - With density set to touch, the drag space never shows up I actually don't want the drag space and currently the only way for me to achieve this is to enable normal density and drag space, then manually un-maximise and maximise the window after every start.